Field service repairs of color television receivers often require the picture tube to be rebiased. Biasing the picture tube is often referred to as setting up the tube. Typical repairs which require a set up include installing a new picture tube, a new signal processing integrated circuit, or a new output device. The initial set up for each receiver is performed in the manufacturing facility, and involves the use of automated test equipment designed specifically for the task. Television manufacturers generally provide some means for field service set up because field service technicians have no access to such specialized equipment.
The field service set up means, referred to above involves the use of a "service line", generated by collapsing the vertical scan to produce a display comprising one, or only a few, horizontal lines running across the approximate center of the picture screen. Defeating vertical scan is not sufficient, by itself, for producing a service line. Unless internal video signal processing is defeated, or unless a specific alternate video signal is provided, the video content of the line will cause undesirable intensity fluctuations of the service line. Ideally, the intensity of the service line should be constant and its color should be "white".
Prior set up arrangements employing a service switch for entering a set-up mode are known from U.S. Pat. No. 3,820,155, entitled TELEVISION RECEIVER SERVICE ADJUSTMENT SYSTEM (Neal), U.S. Pat. No. 3,959,811, entitled SET-UP ARRANGEMENT FOR A COLOR TELEVISION RECEIVER (Shanley, II), and U.S. Pat. No. 4,123,776, entitled SERVICE SWITCH
It is not very practical to require a specific video signal to be substituted for the normal video signal because the service technician might not have access to equipment for producing the required signal. A more practical approach involves defeating the video detector output of the intermediate frequency amplifier (IF amp), or attenuating the video signal by reducing the video gain by affecting the contrast control, or both.
Unfortunately, in many cases it is no longer possible to break the signal path at video detector output, because that point may be inside a very large scale integrated circuit (VLSI), and therfore be inaccessible. One may feel that the desired circuit point could be made accessible by bringing it out of the IC (integrated circuit) by way of a terminal pin. However, this approach is impractical because IC pins are thought of as a precious commodity, there never being enough of them.